有序介质中二苯甲酰甲烷的酮醇互变异构

用紫外光谱研究了三苯甲酰甲烷在β-环糊精及离子型和非离子型胶束介质中的酮醇互变异构性质,β-环糊精的引入或胶束的形成使酮醇异构平衡向烯醇式方向移动.结果表明,用二苯甲酰甲烷的酮醇互变异构性质能够表征有序介质的微环境性质,确定表面活性物质的临界胶束浓度.     

Keto-enol and enol-enol tautomerism in trifluoromethyl-β-diketones

The keto-enol (K?E) and enol-enol (E?E) equilibria of a variety of trifluoromethyl-β-diketones were investigated using ¹H, ¹³C, ¹⁹F NMR spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence indicates that trifluoromethyl-β-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet spectrophotometry show the E?E equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are noted and discussed.     

Keto-enol tautomerism and the dipole moment of the association in the cyclohexanone + carbon tetrachloride mixture

Summary A measuring method and an apparatus have been developed for the determination of the dipole moment of the keto-enol association complex formed in cyclohexanone and in its mixtures. It was stated that both the dipole moment and the amount of the association can be determined by the measurement of the static relative permittivity and the Piekara constant in mixtures of cyclohexanone with nonpolar components. It was found that the association is practically nonpolar [ =1.18·10^–30 Cm (0.35 D)], and the enol formation in the ketone increases with the amount of the diluting nonpolar component. The modelling of the mixture was also made in the form of an A + A₂ + B type ternary mixture using the vapour-liquid equilibrium data of the cyclohexanone + carbon tetrachloride mixture. The degree of enol formation was calculated on the basis of the excess Gibbs free energy of mixing and the static relative permittivity. It was found that the results obtained exclusively from dielectric data ( and ) show a good agreement with those calculated from the thermodynamic model.

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Keto-Enol-Tautomerie und das Dipolmoment des Assoziats im Cyclohexanon-Tetrachlorkohlenstoff-Gemisch

Zusammenfassung Es wurde eine Meßmethode und ein Gerät zur Bestimmung des Dipolmoments des Assoziationskomplexes des sich in Cyclohexanon und dessen Gemischen bildenden Keto-Enols ausgearbeitet. Es wurde festgestellt, daß durch das Messen der statischen relativen Dielektrizitäts-und Piekara-Konstanten sowohl das Dipolmoment des Assoziats, als auch dessen Menge in den Gemischen des Cyclohexanons mit apolaren Komponenten bestimmt werden können. Man stellte fest, daß das Assoziat praktisch apolar ist [ =1.18·10^–30 Cm (0.35 D)], und daß sich die Enolisation im Keton durch Verdünnung mit den apolaren Komponenten erhöht. Die Dampf-Flüssigkeit-Gleichgewichtsdaten des Cyclohexanon-Tetrachlorkohlenstoff-Gemisches wurden berücksichtigt und das Gemisch auch als ein Ternärgemisch A + A₂ + B modelliert; so konnte die Enolisation aufgrund des Überschusses der Freien Enthalpie bei der Vermischung und der statischen relativen Dielektrizitätskonstante berechnet werden. Es wurde festgestellt, daß die aus den dielektrischen Daten berechneten Resultate gut mit den aufgrund des thermodynamischen Modells berechneten Resultaten übereinstimmen.

     

Tautomerism of azine derivatives. 4. Keto-enol tautomerism of β-keto esters of the azine series

The structures of pyrazinoyl-, 3-pyridazinoyl-, 4-pyrimidoyl-, and 2-, 3-, and 4-pyridoylacetic esters were studied by means of IR, NMR, and¹H and¹³C NMR spectroscopy and quantum-chemical calculations (Pariser-Parr-Pople and CNDO/2). The effect of solvents (including strongly and weakly basic solvents) on the position of the tautomeric equilibria of these β-keto esters was studied. The o⁺ constants for the keto and enol fragments were estimated by means of quantum-chemical calculations and¹³C NMR spectroscopy. See [1] for communication No. 3. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 822–826, June, 1980.     

Thermal degradation of poly(parabanic acid)

A variety of techniques were used to follow the thermal degradation of a poly(parabanic acid) (PPA-M). The kinetic data associated with the weight loss as a function of time at elevated temperatures indicates a random initiation process followed by depolymerization. Infrared and mass spectral techniques further confirmed the kinetic data by showing the presence of isocyanate and amide groups in addition to CO, CO₂, and nitric oxide gases being evolved. Incipient gelation occurs simultaneously with the formation of the amide. A reverse Hoffman rearrangement mechanism has been proposed which appears to be consistent with the available experimental data.     

Keto-enol tautomerism of 4-hydroxy-3-phenyl-2-(2-chloroacetamidophenyl)-1-isoquinolone

Three crystalline modifications of the N-chloroacetyl derivative were obtained in the chloroacetylation of 2-(2-aminophenyl)-4-hydroxy-3-phenyl-1-isoquinolone. It was established by IR spectroscopy that they differ with respect to isomeric composition and the character of the intermolecular hydrogen bonds. For the first time, both keto-enol tautomeric forms were isolated in individual form in the 4-hydroxy-1-isoquinolone series.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 542–543, April, 1985.     

On the chemical and electrochemical one-electron reduction of peroxynitrous acid

Peroxynitrous acid was reduced by cathodic linear sweep voltammetry at a gold electrode and by iodide at pH 3.2 and 5.6. The cathodic reduction wave was identified by measuring its decay in time, which was the same as observed by optical spectroscopy. The iodide oxidation was followed by optical measurement of the triiodide formation. Both reductions show one-electron stoichiometry, with the product n(alpha)alpha = 0.23 +/- 0.04 from the electrochemical experiments, in which alpha is the transfer coefficient and n(alpha) the number of electrons transferred, and an diiodine yield of ca. 0.5 equiv per equivalent of peroxynitrous acid. The voltammetric reduction was irreversible up to scan rates of 80 V s(-1). Both reductions were pH independent in the range studied. The voltammetric reduction is most likely an irreversible elemental reaction followed by a chemical decay that cannot be observed directly. Because of the pH independence, we conclude that both reductions have a common short-lived intermediate, namely [HOONO]*-. We estimate the electrode potential of the likely ONOOH/ONOOH*- couple to be larger than 1 V. The commonly used electrode potential E degrees (ONOOH, H+/NO2*, H2O) does not describe the chemistry of peroxynitrous acid.     

The electrochemical reduction of perezone in the presence of benzoic acid in acetonitrile

An electrochemical study using transient techniques of a quinone-type natural product, perezone, has been performed in acetonitrile and in the presence of benzoic acid. Using linear sweep voltametry and single potential step chronoamperometry, it was possible to establish that the reduction mechanism of perezone involves a monoelectronic charge-transfer step, followed by a protonation step and homogeneous charge transfer due to disproportionation of the protonated intermediate. The mechanism for the homogeneous charge-transfer step was found to be of the type DISP1 (disproportionation order one) from the results of double potential step chronoamperometry experiments. The occurrence of the DISP1 mechanism was provoked by the mildly acidic medium used in this study.     

The electrochemical reduction mechanism of trivalent chromium in the presence of formic acid

The electrochemical reduction process of trivalent Cr in the presence of formic acid is studied. The compositions of Cr complexes in electrolytes and products on cathode are investigated using ultraviolet–visible absorption spectroscopy (UV–Vis) and X-ray photoelectron spectroscopy (XPS), respectively. The geometric structures of the original and transition state ions during the electroreduction process are optimized, using density functional theory with General Gradient Approximation/Perdew-Wang 91 (GGA/PW91) calculation. The trivalent Cr primarily exists in the form of [Cr(H₂O)₆]³⁺ in the solution. [Cr(H₂O)₆]³⁺ exhibits regular-octahedron structure which is unfavorable for center Cr³⁺ to contact cathodic electrons. In the presence of formic acid, formate ion promotes the formation of the reactive intermediate, [Cr(H₂O)₄CHOO]²⁺, which possesses irregular-octahedron structure with Cr³⁺ ion as a vertex. In this case, Cr³⁺ ion can contact the cathode and then obtain electrons easily.     

Influence of ascorbic acid on the electrochemical reduction of oxygen on solid electrodes

The influence of ascorbic acid on the stepwise process of reduction of oxygen (molecular oxygen, hydroxyl radicals, hydrogen peroxide) on electrodes of iron, copper, and platinum was established. Hypotheses with regard to the mechanism of the antioxidant action of ascorbic acid in biosystems were advanced.Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, I Murmanskaya Ulitsa, 252660 Kiev-94, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 2, pp. 124–127, March–April, 1996. Original article submitted June 23, 1995.     

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

Photodissociation experiments have been performed for the parabanic acid (C₃H₂N₂O₃) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/aug-cc-pVTZ level of theory.     

Nmr spectroscopic studies on the tautomerism in tenuazonic acid analogs

Nmr spectra of structural analogs of tenuazonic acid such as 3-acetyltetramic acid, 3-acetyltetronic acid, 3-acetylthiotetronic acid and others were investigated for elucidation of the tautomeric structures. These compounds have completely enolized β,β′-triketone systems, and the position of the nmr signals for the enolic proton shows that the strength of their intramolecular hydrogen-bonding is weaker than those of acyclic β,β′-triketones and six-membered cyclic triketones. The assignment was made for nmr signals split by the difference of the diamagnetic anisotropic effect in each tautomers. The percentages of each of the tautomers were calculated from the intensities of the corresponding nmr signals. The results were confirmed by means of ¹³C-nmr spectroscopy.     

Theoretical study on the isomerization and tautomerism in barbituric acid

Isomerization and tautomerism of 16 isomers of barbituric acid (BA) were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a), imaginary frequencies (υ), and Gibbs free energies (ΔG ^#) of the amine-imine and keto-enol tautomerisms and O–H internal rotations were calculated. The activation energies of amine-imine tautomerisms were in the range of 110–200 kJ/mol and for keto-enol tautomerisms were larger than 200 kJ/mol. The calculated activation energies of internal O–H rotations were smaller than 60 kJ/mol. Effect of micro-hydration on the transition state structures and activation energies of the tautomerisms were also investigated. Water molecule catalyzed the tautomerisms and decreased the activation energies of both the amine-imine and keto-enol tautomerisms about 100–120 kJ/mol.     

Radical intermediates in the electrochemical reduction of alizarin

Conclusions In the electroreduction of alizarin in an aprotic medium the stepwise formation of the anion-radical and subsequently of the dianion has been observed; under conditions of the prolonged process the anion-radical disproportionates and through intermediate stages of deprotonation and reduction is converted into the stable dianion-radical.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 983–988, May, 1985.     

α-羰基环十二酮的酮-烯醇互变异构中的构象效应

利用¹H NMR技术研究了α-羰基环十二酮的酮-烯醇互变异构. 结果表明, 取代基、温度和溶剂对互变异构的影响与已知的规律一致, 但是烯醇的含量与相应的α-羰基环己酮相比要低得多, 利用构象效应对其进行了合理的解释.     

Kinetics and mechanism of formation of polymeric products during the hydroxyalkylation of parabanic acid with oxiranes

The kinetics of the reaction between N‐(2‐hydroxyalkyl) hydroparabanates (HAHP) and N,N′‐bis(2‐hydroxyalkyl) parabanates (HAP) with ethylene and propylene oxides were studied. The addition of oxiranes to imide function of HAHP led to the formation of HAP. Further reaction of HAP with oxiranes resulted in trioxoimidazolidine ring opening and polymeric products. The kinetics of the system was studied in detail. Based on the final kinetic equations, the mechanism of the reactions was postulated. The temperature dependences of kinetic parameters and analytical data provided experimental evidences supporting the mechanism. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 399–406, 2006     

Pyrimidine-pyrimidylidene tautomerism of some 4-pyrimidylmalonic acid derivatives

2,6-Diphenyl-4-pyrimidylmalonic esters (I, II) 2,6-diphenyl-4-cyanoacetic ester (III), and 2,6-diphenyl-4-pyrimidylmalononitrile (IV) were obtained for the study of pyrimidine-pyrimidylidene tautomerism by condensation of 4-chloro-2,6-diphenylpyrimidine with the appropriate malonic acid derivatives. The structures of the tautomeric forms and the positions of the equilibria were studied by PMR, IR, and UV spectroscopic methods.Communication LVI from the series Pyrimidines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 395–397, March, 1977.